As is well known, an oscillator is an electronic device that provides a source of alternating current. The frequency of the alternating current that is generated by the oscillator is dependent on the characteristics of various components in the oscillator, and may be any frequency. For a stable type oscillator, in which the frequency of a substantially sinusoidal output is determined by a line or tuned circuit, the oscillator uses the principles of positive feedback. One example of such an oscillator incorporates a standard transistor with positive feedback from the collector of the transistor to its base. The oscillations of this oscillator are tuned, or determined, by a grounded crystal oscillator which is connected into the emitter of the transistor. The output of this conventional oscillator is taken from the collector of the transistor.
In the operation of a conventional oscillator as described above, the use of high power drive levels has several advantages. Perhaps the most apparent advantage is that high power drive levels prevent noise degradation in the oscillator by ensuring a relatively high signal to noise ratio. This high drive power will result in a low floor of the oscillator's phase noise spectrum with a resultant improvement is short term stability. On the other hand, there are significant disadvantages associated with the use of high power drive levels.
During a cold power up of a conventional oscillator as described above, high power necessarily causes the crystal to heat up. This generates severe frequency pulling, or "chirping", at turn on which continues until the crystal temperature stabilizes. An adverse consequence of this is that there can be unacceptable response delays during the time interval the oscillator is stabilizing. Further, at the drive levels associated with high power operation, there can be excessive frequency drift after stabilization.
To avoid the problems of "chirping" and frequency drift, the obvious alternative is to operate the oscillator at low power drive levels. It happens, however, that for a conventional oscillator there are generally unacceptable consequences in oscillator operation at low power drive levels. Importantly, because the resistance of the crystal must be higher than the resistance of the transistor's emitter, the power dissipation of the crystal is still relatively high. Furthermore, the noise floor is actually raised by the lower power level and the loaded Q of the crystal in the circuit is degraded. Due to the adverse effects encountered in low power operation of a conventional crystal oscillator, several factors have gone unattended. For example, the problem of frequency pulling at turn on has not been addressed, and the possibility of optimizing the crystal impedance for best Q of the circuit has not been considered.
In light of the above, it is an object of the present invention to provide an oscillator which reduces the power requirement necessary for crystal oscillator drive, through impedance transformation, to achieve low phase noise while minimizing the frequency pulling that occurs immediately after D.C. power is applied to the oscillator. Another object of the present invention is to provide an oscillator which achieves relatively low crystal power dissipation with minimal degradation in the Q of the crystal. Still another object of the present invention is to provide an oscillator which uses the resonator element as a single band pass filter to reduce the wideband oscillator noise. Yet another object of the present invention is to provide an oscillator which uses the impedance of the resonator to provide a frequency dependant load to the input of the buffer stage so that wide band noise is not contributed by the buffer stage. Another object of the present invention is to provide an oscillator which diminishes the long term frequency drift of the oscillator. It is also an object of the present invention to provide an oscillator which is simple to use, relatively easy to manufacture and which is comparatively cost effective.